Final Exam Fall 2012 #8

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Juskarun Cheema
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Joined: Fri Sep 25, 2015 3:00 am

Final Exam Fall 2012 #8

Postby Juskarun Cheema » Sun Dec 06, 2015 11:49 am

Question: An approximation to finding the pH of a solution containing a polyprotic acid is to treat each step independently and add the [H+] generated at each step. Using this apprximation find the pH of a 0.0037 M solution of H2CO3 using the given equations:
H2CO3 -> H+ + HCO3- Ka1 = 4.3 x 10^-7
HCO3- -> H+ + (CO3)2- Ka2= 5.6 x 10^-11

Why do we have to use two separate equations to solve for pH. Why not use the Ka total?

Posts: 19
Joined: Fri Sep 25, 2015 3:00 am

Re: Final Exam Fall 2012 #8

Postby JuliaPark2H » Sun Dec 06, 2015 2:14 pm

I'm sure using the Katotal is acceptable in solving the problem, but since the problem specifically states the method of how it wants the approximation of the answer, I'm guessing that's why the answer shows how the answer is derived from following the step-by-step method

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